This invention relates to a method and apparatus for printing an image and, more specifically, relates to a printing head for thermally printing an image and a printing method using the printing head.
As an example of a thermal printing apparatus, there is known a color copying machine of the thermal transfer type which reproduces a color image by using thermal transfer ink ribbons having a plurality of colorants. In a conventional apparatus of this kind, the ink ribbons are positioned opposite a copy paper, and are heated selectively by a thermal printing head which has a plurality of heating elements arranged in a line on a ceramic substrate and driving circuits disposed on the ceramic substrate in the form of a micro-package. In operation, the driving circuits provide a set of driving pulses according to the image to be printed and thus the selected heating elements are energized so that the colorants are transferred to the copy paper to form the image. Examples of such prior art machines are shown, for example, in U.S. Pat. Nos. 4,067,017; 4,378,566; and 4,427,985 and European Patent Application No. 0,050,481.
In the printing apparatus mentioned above, however, as all the colorants have the same melting point, the apparatus may not be utilized to obtain an intermediate tone or sufficient gradation. It is, therefore, known to obtain gradations in the hard copy by controlling the image density by varying the application time of the driving pulses to the thermal printing head, or by controlling the image density of printed dots by varying the number of driven heating elements assigned to form a picture element. However, the relationship between the image density and the application time of the driving pulses is not linear; thus, if a smooth gradation is to be achieved by varying the application time, complicated conditions must be met. Moreover, one picture element is defined by a plurality of recording dots. Therefore, the resolution of the printed image deteriorates when a smooth gradation is obtained by varying the number of driven heating elements.